The invention relates to processes for manufacturing objects having a plastic substrate, preferably a plastic foam, covered by a polyolefin layer which has improved adhesion to the plastic substrate.
Polyolefin plastics are more or less inert because of the structure. Polyolefins, such as polyethylene and polypropylene, are produced on a very large scale. Comparatively, polyethylene is more inert than polypropylene and is difficult or impossible to bond to other materials. It adheres hardly or not at all to substrates such as, for example, metal surfaces or foamed plastics. To adhere polyethylene to a substrate, an adhesive must be used, or the surface of the polyethylene or the substrate must be etched. However, adhesives effect only a limited degree of adhesion due to the poor bonding properties of polyethylene. Moreover using adhesives requires extra treatment steps, which correspondingly increases the cost and price. Etching one or both surfaces of the materials being bonded together is a laborous process, which, like the application of adhesives, lengthens production time while increasing the cost and price. Similar difficulties are encountered when polypropylene is used.
It is already known that objects can be made by heating powdered plastic in a mold. The powder melts on the hot wall and and forms a layer. Foam structures can be prepared by introducing foam layers or placing foam against a plastic to improve insulating properties or to limit the weight of the objects. The surface layer and the foamed or unfoamed layer in contact with it should adhere to each other. Particularly with polyethylene and polypropylene as well, the adhesion leaves much to be desired.
This problem has been encountered particularly in the manufacture of surf boards, which are generally made by rotational molding, to form an outer wall of polyethylene. In particular high or medium density polyethylene is employed. High or medium density means at least 0.930 g/ml. However, it is possible to use lower density polyethylenes. The space in between the surface layers is filld with a polyurethane foam resulting in an object having polyurethane foam coated with polyethylene.
The polyethylene coating may get damaged during use. Water can then penetrate into the foam. If there is good adhesion between the surface wall and foam, damage is less likely to occur. With good adhesion, water can only penetrate into the foam under the damaged spot. With poor adhesion between the sirface wall and the foam water can seep between the surface wall and foam and can thus be absorbed by all the foam. Consequently, a need for good adhesion is desired for all polyolefin coated plastics.
Good adhesion is highly desirable for surf boards in particular and for other composite objects built up of similar components.
Improved adhesion of polyethylene to metal substrates by blending the polyethylene with oxidized linear polyethylene is claimed in U.S. Pat. No. 3,639,189. The oxidized polyethylene is obtained by heating linear polyethylene in an oxygen atmosphere at temperatures that may range from about 90.degree. C. up to the crystalline melting point of the polyethylene until the desired degree of oxidation has been reached. Next, the oxidized polyethylene is melt-blended with the oxidized polyethylene, and granulated. The thus obtained granulate is used for the layers applied to a metal substrate. Oxidizing polyethylene is a laborious process and increases the cost and price.
Such already known compositions suffer from several drawbacks. Such compositions are less suited for manufacturing, for example, surf boards, because oxidized polyethylene lowers the resistance of the polyethylene compositon to atmospheric attack. More specifically, the resistance to thermal and oxidative attack is lowered. In particular, it is highly undesirable for an outer wall, such as the polyethylene coating of a surf board which is exposed to outdoor atmospheric influence, to have lowered atmospheric attack and weather resistance. This drawback could be overcome by including more stabilizers in the polyethylene composition, however this further increases the costs and price.
Moreover, using compositions containing oxidized polyethylene to manufacture objects in a mold, for instance by rotational molding, has the further disadvantage that oxidized polyethylene compositions adhere to the mold surfaces. Even if non-oxidized polyethylene is used, release agents must also be applied in order to facilitate the release of the polyethylene object from the mold. When oxidized polyethylene is employed difficulties are still encountered in removing the object from the mold even if release agents are applied.
It has already been attempted to improve other properties in addition to adhesion by applying a cross-linked polyolefin, by preference cross-linked polyethylene, or by cross-linking the polyethylene after it has been formed into a surface layer. This improves adhesion characteristics. However, disadvantageously damage to the surface layer remains difficult or impossible to repair.
It has now been found that improved polyolefin adhesion, in particular polyethylene, to a subtrate can be effected in a mold by producing a surface layer of a stabilized polyethylene in the mold and then employing hardly stabilized or unstabilized polyolefin to form a second layer on the inside surface of the first layer in the mold. The substrate, for example, can be another plastic such as polyurethane foam.